This invention relates to tear-resistant coated abrasive products and, more particularly, to coated abrasive products that have as a backing material a barrier-coated tear-resistant laminate of fabric and paper and to a method of making the coated abrasive product.
Coated abrasive products have been used for at least a century to finish the surfaces of various substrates such as wood, metal, plastic and composite materials. Coated abrasive products typically comprise a flexible backing material which is overcoated with an abrasive layer comprised of abrasive particles and a cured binder material. It is customary to make some coated abrasives by application of a make or maker coat of a binder material to the backing, application of abrasive particles to the make coat while it is sufficiently tacky to adhere to the abrasive particles and then overcoating the make coat containing the abrasive particles with a size coating. The make coating may be partially cured prior to application of the size coating but once the size coating is applied, it is typical to fully cure both the make and size coating so that the resultant coated abrasive product can be employed as an abrasive material. It is sometimes customary to flex the resultant coated abrasive product to impart cracks in the abrasive layer to provide suitable flexibility for the coated abrasive product. Thereafter, the coated abrasive material is converted into various abrasive products, e.g., rectangular sandpaper sheets, circular abrasive discs, or abrasive belts typically by cutting a strip of the coated abrasive product and joining the ends of the strip to provide an endless belt.
Any of a variety of backing materials have been used in the manufacture of coated abrasive products. The selection of backing material is typically made based upon the intended use of the product. Material such as paper, fabric (either nonwoven or woven) plastic film or combinations of these materials have been employed.
Paper backings for coated abrasive products are preferred for many applications because of their relatively stiff, but flexible nature, which makes them suitable for use as coated abrasive sheets, abrasive discs and abrasive belts. Paper, however, has a relatively low tensile strength at break which may cause a coated abrasive product with a paper backing to fail prematurely before its abrasive surface is fully utilized. This is particularly true when the abrasive granules are of a coarser nature which transmits greater stress to the backing as the abrasive product is being employed, particularly as an abrasive disc or abrasive belt.
Other problems encountered with paper backings include surface roughness, particularly where finer abrasive particles are employed and, in the case of abrasive discs, edge chipping where the bond between the paper substrate and the make coat fails before the abrasive coating has been fully used.
A solution to the surface roughness problem is proposed by Darjee, et al. in U.S. Pat. No. 5,286,541. This patent teaches the use of a combination backing for coated abrasive products which includes a flexible paper member having an ionomer film layer adhered to its top surface to which the make coat is adhered to provide a smooth surface to the backing.
European Patent Application No. 0 716 903 A1 discloses a backing material made of a nonwoven fiber mat which is reinforced to form a flat plane wear- and tear-resistant substrate by means of a binder or by superficial dissolving or fusing of the fibers which may be further reinforced by laminating a reinforcing fabric to its backside. This backing does not include paper and, thus, its use would not have the benefits of having a paper backing for a coated abrasive product.
It is known to use commercially available laminates such as the cylinder paper/cloth fabric laminate sold by McKenzie Coating and Finishing Company, Hyde, United Kingdom as a backing for coated abrasives. This laminate is typically provided with an abrasive coating over the cloth side but without first coating the fabric side with a barrier coating. The absence of a barrier coating allows penetration by the liquids in the curable make coating composition into the fibers of the cloth fabric which tends to embrittle the fibers and weaken the laminate. Moreover, even if this laminate were used as a coated abrasive backing with a barrier coat, the resultant backing would not have the needed tear strength in the cross direction to meet the requirements needed to provide a tear resistant coated abrasive product.
Thus, a need exists for a backing for a coated abrasive product which has sufficient tear strength to be utilized in disc and belt applications under any of a variety of conditions which also has the beneficial properties of a paper backing to provide a backing which is relatively stiff, but flexible.
This invention provides a coated abrasive product which includes an abrasive layer applied to a backing comprised of a laminate of paper and fabric. The backing is relatively stiff, but flexible, and has sufficient tear strength to be employed under any of a wide variety of applications including use in abrasive discs and in abrasive belts. The coated abrasive products of the invention also have a backing caliper that is significantly more uniform than their cloth-backed comparatives.
Specifically, the invention provides a coated abrasive product comprising:
a. an abrasive layer;
b. a barrier coating; and
c. a relatively stiff, but flexible, backing comprising at least one fabric sheet and at least one paper sheet adhesively laminated together by an adhesive material to provide a laminate having at least one major surface provided by said fabric sheet that is coated with said barrier coating to which said abrasive layer is adhered and a second opposite major surface provided by said paper sheet, wherein the backing has an Elmendorf tear test value in the cross direction of at least about 10,000 mN.
Preferred coated abrasive products of the present invention have an Elmendorf tear test value in their cross direction of at least 6000 mN.
As used herein, the following definitions shall apply:
An xe2x80x9cabrasive layerxe2x80x9d refers to a layer comprised of abrasive particles adhered within or to a cured binder coating.
A xe2x80x9cfabricxe2x80x9d refers to a woven or nonwoven fabric formed as an integral sheet of fibers that are either woven together or bonded together either by a binder or, in the case of thermoplastic fibers, by heat and/or pressure, solvent treatment or by other methods without adhesive.
A xe2x80x9cbarrier layerxe2x80x9d refers to a layer of a cured composition which has an initial liquid state which will penetrate the fabric sheet and in the liquid state is a non-fiber embrittling coatable composition comprised of binder in solution or in a dispersion which, on curing, results in a cured composition that is substantially impervious to subsequent liquid-based coatings applied thereover.
A xe2x80x9cnon-fiber-embrittling coatable compositionxe2x80x9d is a coatable composition that has ingredients which have only a minimal or no embrittling effect on the fibers of the fabric sheet.
A xe2x80x9cspunbonded fabricxe2x80x9d refers to a fabric formed of filaments that have been extruded, drawn, laid on a continuous belt and then immediately bonded.
The fabric sheet may be a woven cloth, but preferably it is a nonwoven fabric such as a spunbonded nonwoven web. The fabric preferably has a thickness of about 0.1 to about 0.4 mm. The fabric preferably has a basis weight in the range of about 17 to 230 grams per square meter (gsm). Useful cloth fabrics have a tensile strength at break of at least about 50 kg/25 mm.
The backing may include a second fabric sheet adhesively laminated to its second opposite major surface to provide further reinforcement of the backing. This is particularly desirable where large abrasive granules are to be employed in the abrasive layer or for high throughput grinding. The second fabric may be a woven fabric, like the first fabric, but preferably is a nonwoven fabric such as a spunbonded nonwoven web.
The adhesive material employed to laminate the fabric to the paper is preferably a thermoplastic adhesive such as an ionomer adhesive material and, most preferably, is a zinc salt of ethylene methacrylic acid copolymer.
Any of a variety of papers customarily employed as coated abrasive backings may be employed to provide the paper sheet. Preferably, the paper sheet is a cylinder paper which preferably has a basis weight in the range of about 100 to 400 gsm. Useful papers preferably have a machine direction tensile strength at break of at least about 40 kg/25 mm and in the cross direction of at least about 16 kg/25 mm.
The barrier coating composition is preferably a reactive acrylic latex with carboxyl functionality which cures to provide a crosslinked acrylic resin.
Most preferably, where the coated abrasive product is to be utilized for finishing wood products where charged particulate airborne wood particles are generated, additives may be included to make the coated abrasive product electrically conductive to dissipate the electrical charge. Such additives may be added to the barrier coating composition, for example, in the form of carbon black particles or to the paper, in the form of short carbon filament segments.
The invention further provides a method of making the coated abrasive product. The method comprises the following steps:
a. laminating under pressure a fabric sheet to a paper sheet with an adhesive composition to provide a relatively stiff, but flexible, backing having at least one major surface provided by the fabric sheet and a second opposite major surface provided by said paper sheet, said backing having an Elmendorf tear test value in the cross direction of at least about 10,000 mN;
b. coating said one major surface of said backing with a non-fiber-embrittling curable barrier coating composition;
c. curing said barrier coating composition; and
d. coating said barrier coated major surface of said backing with an abrasive coating.
The preferred method of providing the abrasive coating is by:
a. applying a make coat to said barrier coated major surface of said backing;
b. applying abrasive granules to the make coat before it is fully cured;
c. at least partially curing the make coating;
d. applying a size coating to the abrasive granule coated make coat; and
e. fully curing the make and size coats.
The coated abrasive product may be converted by utilizing conventional techniques into coated abrasive belts, coated abrasive discs and rectangular coated abrasive sheets.